The invention relates to a graphite tube and related elements for an atomizing device, which are used in flameless atomic absorption spectroscopy.
An atomizing device is known from German patent specification 2 314 207 in which the graphite tube is heated by passing an electrical current therethrough. An inert gas flow is directed inwardly from the ends of the tube, through the tube, and discharged through a transverse bore. In this prior art atomizing device, the transverse bore is located in the transverse central plane, which is exactly halfway between the ends of the graphite tube. It will be appreciated that this center is also the hottest portion of the graphite tube, as the ends thereof are in heat exchange relationship with the electrodes or contact pieces, to which the current to the graphite tube is conducted, and the electrodes are mounted in cooling jackets. As a consequence, the inert gas flow is directed from the cooler surface areas of the graphite tube to the hotter ones to prevent substances carried by the flow from being deposited on the cooler surface areas.
Furthermore, it is known from U.S. patent application Ser. No. 608,558, dated Aug. 28, 1975, now U.S. Pat. No. 4,022,530, to make the electrodes tubular, so that they together surround the graphite tube like a jacket. According to this patent specification, one of the electrodes is longer than the other one, so that the parting line therebetween is unsymmetrical with respect to the center of the graphite tube. The electrode, which is longer, has a transverse bore aligned with the transverse bore of the graphite tube.
In practice, the prior art arrangements present some problems. By providing the transverse bore for the feeding of the sample in the center of the graphite tube, there is a mechanical weakening of the graphite tube in its hottest zone, thereby reducing the useful life of the graphite tube. In addition, the gas flow tends to remove graphite particles from the transverse bore in this hottest zone. This is particularly true when, after the drying, decomposing and atomizing of the sample, the tube is heated to its maximum temperature, with maximum inert gas flow, in order to prepare it for the next measurement.
As another technical problem, it is difficult to achieve equal gas flow from both ends of the tube, and hence, accurately adjusted flow producing means must be provided. Further, the placing of a liquid sample at the maximum of the temperature profile can result in splitting-up of the sample and sputtering during vaporization, because the sample is heated to its greatest extent at its center and starts vaporizing from that point.
In the arrangement of U.S. patent application Ser. No. 608,558, now U.S. Pat. No. 4,022,530, the heat dissipating from the two jacket-shaped electrodes to the cooling jackets differ, because they receive different proportions of the radiation from the graphite tube, due to their differences in length. In addition, the requirement of having electrodes of different lengths increases the manufacturing and storage costs.